[Haskell-cafe] Is this haskelly enough?

[Johan Tibell]

Sounds like what I want. I'll give it a try. Thanks.

On 7/18/07, Tillmann Rendel <rendel at rbg.informatik.tu-darmstadt.de> wrote:
>
> Johan Tibell wrote:
> > I found myself wanting a map that looks at neighboring elements. This is
> > where I used explicit recursion the most. Something like this:
> >
> > f [] = []
> > f ((Foo a) : (Bar b) : xs)
> >   | fooBar a b = Foo a : f xs
> >   | otherwise = Bar b : f xs
> >
> > This is almost a map. A variation is when filtering and you want some
> > look-ahead to make the filtering decision. There's probably a good way
> > to do this I'm not aware of.
>
> If you want to map over all elements, but need to look ahead in the
> mapped function, you can map over the tails:
>
>    map' :: ([a] -> b) -> [a] -> b
>    map' f = map f . tails
>
> f should be something like
>    f (a:b:c:_) = ...
>
>
> If you want to handle groups of n elements together, producing only one
> element per group, you can use unfoldr with splitAt:
>
>    map'' :: Int -> ([a] -> b) -> [a] -> [b]
>    map'' n f =
>      map f . unfoldr (((not . null . fst) `guarding`) . splitAt n)
>
>    guarding p x = guard (p x) >> return x
>
>
> If you want to decide in the mapped function how many elements to
> consume, you can use unfoldr directly.
>
>    Tillmann Rendel
>
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